Aqueous coating compositions are well known and find use as protective coatings for various substrates such as wood, stone, plastic and the like. Such compositions typically contain a film-forming polymer such as an acrylic polymer. That polymer forms a continuous protective coating over the substrate, often in conjunction with other additive materials such as other polymers, resins, waxes, pigments, dyes, ultraviolet light absorbing agents and the like, to help protect it from damage caused by the sun, abrasion, foot traffic, pressure, soiling, and so forth. The film-forming polymers are typically selected for their physical properties such as water resistance, flexibility, removability, hardness, resilience, toughness, scratch and marking resistant (black heel marks) to fit the needs of the substrate being protected and the environment in which the coating will be used.
A major impediment to good coating or film formation is low temperature during application of the composition. The film forming polymers used in floor coating compositions normally have minimum film formation temperatures (MFT) well above room temperature. Typically, plasticizers and coalescents are used to reduce the MFT of the finish so that film formation will occur at floor surface temperatures as low as 50° F. (10° C.). If the temperature of the floor falls below the MFT of the finish, a properly fused film will not form. There have been many cases where a product performed well on the upper floors of a building, but the cooler ground floor exhibited a powdering problem. The phenomenon known as evaporative cooling will also reduce the temperature of the floor as the finish dries.
A plasticizing agent is typically a compound or a mixture that can associate with the polymer and thereby modify the physical properties of the polymer or of the coating composition itself. For example, a plasticizing agent may serve to change the hardness, flexibility, glass transition temperature (Tg) to form a continuous film. A coating composition without plasticizers may dry to a powder and become useless. Therefore, the amounts and types of the various plasticizers required to produce an optimized floor finish film are of critical importance. Of course, other factors also affect the film formation process, such as temperature, humidity and nature of the substrate. The right plasticizing agent in the right amount imparts desirable properties to the film left behind when the coating composition is applied to the substrate and the volatile components are removed such as by drying. These desirable results are achieved by the plasticizing agent without having to change the nature of the coating polymer itself or having to obtain additional properties not possible from modification of the coating polymer itself.
There are two types of plasticizers: permanent and fugitive. The fugitive plasticizers are also referred to as coalescents. Both types of plasticizers are required in a balanced floor finish and have a significant effect on the performance of a film. Permanent plasticizers are absorbed completely by the polymer and other solid components upon drying and remain in the film to provide continuing plasticization. Coalescents, which volatilize during the drying process, are used to temporarily lower the MFT of a coating composition during the drying phase. The evaporation rate of the fugitive plasticizers or coalescents must be carefully evaluated, so that proper film formation will be accomplished before the coating composition has completely dried. If the evaporation rate of the coalescents is too slow, the film will remain soft and even tacky after drying.
A permanent plasticizer of special significance in floor coating compositions is tributoxyethyl phosphate (TBEP). As a result of its hydrophobicity (solubility in water is 0.2%), TBEP resides on or inside the polymer particles in a wet composition. With a solubility parameter of 8.7 TBEP is predictably a good solvent for acrylate-styrene copolymers, however, it also acts as an effective leveling agent, for reasons not well understood.
When removing a floor coating composition containing tributoxyethyl phosphate, the waste water contains phosphorus, an environmentally undesirable contaminant. Therefore, there is a need for coating compositions using less phosphorus as plasticizers.